Long Lulls Could Help Predict Impending Quakes

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Evidence is growing that powerful earthquakes are preceded by a
years-long lulls in seismic activity, suggest new analyses of
major quakes, including the massive temblor that struck Japan in
March.

Learning more about such lulls could help scientists predict when
disaster might strike, said seismologist Kei Katsumata at
Hokkaido University in Japan. He predicts that an
area near Tokyo could soon be hit by a devastating magnitude
9 quake.

For decades, scientists have suggested there were calm periods
before moderate to major earthquakes, lulls where there were
fewer smaller quakes than normal for anywhere from 15 to 75
months, presumably because
stress was building up in faults. Still, it remains unclear
how common these lulls are, what characteristics they have and
what might be the best way to measure them.

To learn more about such
earthquake lulls, Katsumata investigated the 2003 Tokachi-oki
quake. This magnitude 8.3 temblor off the Pacific coast of the
Japanese island of Hokkaido caused extensive damage, landslides
and power outages, and there were no precursory tremors seen
beforehand as detected with some other earthquakes.

Katsumata analyzed nine years of seismic activity preceding the
2003 earthquake, involving about 2,000 temblors of magnitude 3.3
and higher in the Hokkaido region. He discovered lulls where
seismic activity dropped 42 to 49 percent in the four to five
years before the main shock occurred at two locales on or near
where the fault ruptured.

These lulls were of similar length to several lulls noticed
before other major earthquakes, such as the magnitude 7.9
Kermadec earthquake (which hit the islands of the same name
northeast of New Zealand) in 1976, the magnitude 7.9 Andreanof
Island (part of Alaska's Aleutian island chain) earthquake in
1986 and the magnitude 8.3 Hokkaido Toho-oki earthquake that hit
Japan in 1994. This research, detailed online Oct. 15 in the
Journal of Geophysical Research-Solid Earth, suggests that a calm
or quiescence of five years before magnitude 8 temblors might be
something researchers want to keep an eye out for in the future.

"I think that seismic quiescence is the most promising method for
intermediate-term earthquake prediction, 'intermediate-term'
meaning some years long," Katsumata told OurAmazingPlanet.

Future threat

Katsumata also investigated the devastating
magnitude 9.0 quake that struck off the coast of Tohoku in
Japan in March. He analyzed 5,770 earthquakes with magnitudes 4.5
or greater that occurred in Japan between 1965 and 2010 and found
there seemed to be a lull in seismic activity in the Tohoku area
beginning in 1987, findings he detailed online Sept. 27 in the
journal Earth, Planets and Space.

"I have a hypothesis that a seismic quiescence lasting more than
20 years is a precursor to magnitude 9 earthquakes," Katsumata
said.

This research also revealed a seismic lull in the Boso area of
Japan.

"If my hypothesis is correct, we will have another magnitude 9
earthquake in Boso area," Katsumata warned. "The Boso area is
very close to Tokyo."